1. Field of the Invention
The present invention relates to vehicle braking systems, and in particular to electronic braking systems (EBS) which have a back-up braking facility. The invention is particularly suited to rear axle braking systems for a wide range of vehicles including trailers.
2. Discussion of the Background
With the introduction of electronic braking systems, there has arisen a need for a pneumatic backup system. The introduction of electronic control of the braking pressure in conjunction with the introduction of electrical measurement of axle load by suspension height and/or air suspension pressure has led to the removal of rear axle load valves. This A results in the situation that under failed conditions of the electronic braking system, such that the pneumatic back-up system is in operation, there is no load apportioning of the pressure to the rear brakes, which can lead to vehicle instability under braking at lower vehicle loads.
Existing braking systems (e.g. Great Britain Patent No. 2235505) use a load apportioning valve that is connected with a mechanical link to the axle in order to provide a load regulating input to the valve. The valve is positioned between the braking source and the brake actuator.
It is an object of the present invention to provide a simple method of incorporating load apportioning into the braking system only when back-up braking is required.
In accordance with the present invention, a braking system for a vehicle comprises a primary electronic braking circuit;
a back-up braking circuit having a source of back-up braking pressure;
a braking actuator for braking a wheel on a vehicle axle; and
a changeover valve which selectively applies one of the primary and back-up braking pressures to the braking actuator during braking;
characterised by a regulating valve located in the back-up braking circuit between the source of back-up braking pressure and the changeover valve and being operable as a function of axle loading and backup braking pressure to regulate the supply of back-up braking pressure to the changeover valve.
During normal braking the changeover valve is actuated to connect the primary braking circuit to the braking actuator. When the primary braking circuit is defective, the changeover valve connects the back-up braking circuit to the braking actuator.
The provision of a regulating or apportioning valve located in the back-up braking circuit between the source of back-up braking pressure and the changeover valve provides a simple method of load apportioning into the braking system under back-up braking conditions.
In a preferred embodiment, the regulating or apportioning valve is actuated by air suspension pressure, which gives a signal which is representative of the axle load. The apportioning valve thus controls the application of braking pressure and restricts the application of braking pressure if the axle load is too low, either as a result of light loading or as a result of braking.
Alternatively, a mechanical linkage which varies in accordance with suspension height and axle loading may be used to operate the apportioning valve.